The present invention relates generally to metal alloy compositions, and more particularly to corrosion-resistant ordered intermetallic iron-aluminide alloys, which exhibit improved weldability while maintaining their mechanical properties, in particular, iron-aluminide alloys possessing better hot-cracking resistance as compared to previous alloys.
Iron-aluminides (particularly FeAl-type alloys with &gt;30 at. % Al) have been found to be more resistant to many forms of high-temperature oxidation, sulfidation, exposure to nitrate salts and other corrosive environments than many iron-based corrosion-resistant Fe--Cr--Ni--Al alloys or nickel-based superalloys. In the past, the use of FeAl-type iron-aluminide alloys has been limited by their low ductility and brittleness at room-temperature, poor high-temperature strength above 600 .degree. C., and poor weldability.
It has been observed that generally optimum mechanical properties (including room-temperature ductility, and high-temperature tensile-yield and creep-rupture strengths) of Fe.sub.3 Al and FeAl type iron-aluminides do not generally coincide with optimum weldability. One measure of relative weldability has been to qualitatively describe whether or not cracking occurs during unrestrained welding ( hot-cracking ), but recently, a testing device (Sigmajig) has been developed that quantitatively determines hot-cracking susceptibility of alloys and metals by measuring the threshold cracking stress (.sigma..sub.o) obtained by restrained welding with different applied stresses. There is a need for improved weldability to enable the use of FeAl alloys which have exceptional corrosion resistance in place of conventional structural materials, such as stainless steel. There also is a need for improved weldability of FeAl alloys to make them suitable for structural applications compared to less weldable iron-aluminide alloys. Such structural applications also require that the FeAl alloys possess improved mechanical properties such as high tensile strength and low creep rates. In addition, there is a need for improved weldability of FeAl alloys so that such alloys can be used as filler-metals to weld and join other FeAl type alloys that are useful for structural applications. Such improved FeAl alloys may be useful as an inherently corrosion-resistant weld-overlay cladding on a different structural metal substrate.
Accordingly, it is the object of the present invention to provide an improved FeAl-type metal alloy composition.
Another object of the invention is to provide an improved alloy of the character described that has improved weldability.
It is another object of the invention to provide a weldable alloy of the character described that has acceptable resistance to oxidation, sulfidation, molten nitrate salt corrosion and other forms of chemical attack in high-temperature service environments.
Another object of the invention is to provide a weldable alloy of the character described which also provides an acceptable combination of oxidation/corrosion resistance and mechanical properties.
A further object of the invention is to provide a weldable alloy of the character described which also exhibits sufficient high-temperature strength and fabricability for structural use.
Still another object of the invention is to provide improved weldability of FeAl-type iron-aluminide alloys of the character described for use as weld filler-metal and as weld-overlay cladding material.
Yet another object of this invention is to provide methods for making weld-consumables for metal compositions having the aforementioned attributes.